1. Field of Invention
This invention generally relates to processing compressed digital images.
2. Description of Related Art
Data compression is required in data handling processes, where too much data is present for practical applications using the data. Commonly, compression is used in communication links to reduce the transmission time or required bandwidth. Similarly, compression is preferred in image storage systems, including digital printers and copiers, where xe2x80x9cpagesxe2x80x9d of a document to be printed are stored temporarily in precollation memory. The amount of media space on which the image data is stored can be substantially reduced with compression. Generally speaking, scanned images, i.e., electronic representations of hard copy documents, are often large, and thus make desirable candidates for compression.
During compression, a scanned image may take a form of a pixel map, where the mapped image is created by digitizing light reflected off of physical media using the scanner. A pixel map is one in which each pixel represents some xe2x80x9cvaluexe2x80x9d which indicates the color of the image, or, in the case of gray scale document, how light or dark the image is, at a particular point. Pixel maps may have values that are taken from a set of discrete, non-negative integers. The term bitmap is used to mean a binary pixel map in which pixels can take one of two values, 1 or 0.
This invention is directed to compressing images from documents which contain different types of data. Mixed Raster Content (MRC) is one approach to satisfying the compression needs of differing types of data. MRC involves separating a composite image into a plurality of planes, and separately applying an appropriate compression technique to each image plane. The document is represented by a pixel map which is decomposed into a three-plane representation of a upper plane, a lower plane, and a binary selector plane that is used to select between the upper and lower planes, for example.
While the MRC technique has shown to be successful at separately processing planes, the segmentation process leaves data in the associated planes in areas that will not be chosen by the selector plane. Thus, the associated planes often will have pixels that are not used to reconstruct the final image. This often results in an increase in the number of bits of useless data that are required to encode the entire image, thereby decreasing the compression ratio.
The systems and methods of this invention improve the performance of the compression process by reducing the amount of data. This invention provides systems and methods that use the selector plane as a reference to aid in reducing the amount of data necessary to encode each of the associated planes.
The systems and methods of this invention provide a smoothing technique used to pre-process the upper and lower planes using the information contained in the selector plane, thereby reducing the amount of data that will be subjected to further processing.
The systems and methods of this invention provide techniques that determine useless data and then smooth the boundary between the actual image data and the useless data. In the systems and methods of this invention, the determined useless data is replaced by values that improve compression.
In various exemplary embodiments of the systems and methods of this invention, useless data in non-image areas of a plane is replaced by a constant value and the boundary regions are smoothed by filtering using a segmented-computing filter structure. In various exemplary embodiments of the systems and methods of this invention, the boundary regions are smoothed independently of the compression techniques to be used. Thus the boundary regions reduce the distortion and increase the compression rate achieved by compressing the planes using generic compressors.
It should be appreciated that the compression systems and methods of this invention can be applied to any set of compression techniques for separately processing the various types of data contained in a composite image. Accordingly, the compression/decompression methods and systems of this invention can be applied to any number of systems, including digital printers and copiers, that need to provide compressed or decompressed images. Systems implementing this invention can include data or image processing systems capable of compressing or decompressing images. The systems and methods described herein are efficient and result in smoother boundaries between regions of different image data types.
These and other features and advantages of this invention are described in or are apparent from the following detailed description of various exemplary embodiments.